In certain circumstances a substance is stored in a container, in which container different materials of the substance are divided into different layers. If the substance comprises two liquid materials which do not mix, such as an oil-based material and a water-based material, the denser material will collect closer to the bottom of the container, and the less dense material will collect closer to the surface. Another example may be a substance comprising a first generally liquid material, into which solid particles of a second material are dispersed. If left to rest, the second material particles will subsequently gather either on top of or under the liquid first material, dependent on which material is denser.
In such conditions it may be of interest to measure the amount of the different materials in the container, or the levels of surface defining the material layers. One example of a situation where such a measurement is of interest is in a container of petroleum oil, which also contains a certain amount of water. This may be the case in a tank of a oil tanker ship, or in an oil cistern e.g. in a refinery. The water may be deliberately disposed in the container, or be present due to condensation. Nevertheless, once placed in the container, the oil and the water will separate, such that the oil is collected in an upper layer and the water in a lower layer. Since the amount of oils is generally considerable larger than the amount of water, a surface dividing the two layers will be disposed close to the bottom of the container, and the actual position of the dividing surface is not easily assessed.
Solutions for measuring the level of water in an oil tank have been provided in the prior art. A problem is that the level of water preferably should be determined with an accuracy of a few millimeters, even when the water layer is located under a layer of oil of up to 35 meters or more, the dielectric constant of which is not well known. It has therefore not been possible to provide a contact free method for determining the level of the water with reasonable accuracy through the oil. Prior art solutions have consequently made use of alternative solutions, such as capacitive sensors applied in a narrow tube extending from the bottom of the container to the top.